For certain applications there is need of polymeric fatty acids which consist completely or for the greater part of trimer and it is therefore attractive to have available a method of preparation which provides high yields of polymeric fatty acid and in which, moreover, the polymeric fraction has a high content of trimer.
From the U.S. Pat. No. 3,097,220 it is already known to prepare polymeric fatty acids having a content of trimer (50-60% trimer), by carrying out the reaction in two steps, namely: in a first step by heating with a Friedel-Crafts catalyst (BF.sub.3) and thereafter in a second step by heating in the presence of water and a clay catalyst. The use of the gaseous, toxic BF.sub.3 gives problems, because the fatty acid/BF.sub.3 mixture is extremely corrosive and requires special reactors (e.g. coated with Teflon). Moreover, the removal of that catalyst is difficult and the catalyst cannot be recovered.
On the other hand it is already known from the Dutch patent application No. 7312712 (=German DOS No. 22 50 470) to dimerize unsaturated fatty acids with the aid of ion-exchange resins which are insoluble therein and which contain sulphonic acid groups. According to said patent application, page 4, lines 23-29, the reaction product thus obtained consists mainly of dimeric fatty acids and only for 10-20% by weight of trimeric and higher polymeric products. Moreover, the yield of polymeric fatty acids is relatively low.
U.S. Pat. No. 3,367,952 describes a similar process, though without indicating what the yields of dimer and trimer are. In our experience this process also yields a relatively small amount of trimer.